Web clipper by default tries to find a running desktop instance on port 37740. If you configured your Trilium desktop app to run on a different port, you can specify it here (otherwise keep it empty).
If you have a server instance set up, you can optionally configure it as a fail over target for the clipped notes. Desktop instance will still be given priority, but in cases that the desktop instance is not available (e.g. it's not running), web clipper will send the notes to the server instance instead.